The mirror glass going brown is a sign that the auto dimming feature of the mirror has failed.
Only option is to replace the mirror glass, you will be shocked with the price!!
Some have fitted non dimming glass in place of the original to save money.
Options on replacement might have increased though as mine failed about 6 years ago so there may be factor part replacements now.
Some say that the liquid if it gets on your paint can damage it. Cant comment on whether this is true.
230k is spot on. And you will be shocked at the replacement price. Mine went on my SL, I got a cheap replacement from Nick, iirc it was about £100. Believe me, that was cheap!